A hybrid vehicle is driven by a drive source that is a combination of an engine and a motor capable of electricity generation (motor generator), and such a vehicle has been developed for improving fuel efficiency and for reducing exhaust gas in environmental protection. Typically, a hybrid vehicle is controlled for stopping the engine when the vehicle stops in so-called idling-elimination. In this case, when the engine stops, the mechanically driven oil pump, which has been in operation for supplying hydraulic oil, for example, to the ratio-change mechanism, also stops. Therefore, for complementarity, an electrically driven oil pump or electrical oil pump is provided to supply hydraulic oil only while the engine is not in operation under the idling-elimination control, the electrical oil pump being driven by an electrical motor, which receives electric power from the battery (refer to, for example, Japanese Laid-Open Patent Publication No. 2003-307271).
The electrical oil pump operates only while the engine is not in operation in idling-elimination control. If air enters the hydraulic circuit as bubbles while the engine is operating, i.e., while the electrical oil pump is not operating, then when the electrical oil pump is restarted, it may suck up the bubbles and skid momentarily, resulting in a delay in the rise of the hydraulic pressure applied by the electrical oil pump.